Neurophysiological studies have shown how disparity-tuned neurons in primary visual cortex make the initial disparity measurements. However, the response of these neurons bears little relation to human depth perception. While the disparity-tuned neurons code absolute disparity, and are little affected the context of stimuli falling within their receptive fields, depth perception depends heavily on relative disparity, and is greatly influenced by the surrounding disparity field. How does this transformation from the disparity signals to the signals that generate perceived depth occur? One aspect of human stereopsis that is probably limited by these primary neurons is stereoacuity. Yet, stereoacuity may also be influenced by stimulus conditions that profoundly affect perceived depth. Stereoacuity can thus serve as a psychophysical probe for how the signals generated by disparity-tuned neurons are combined in subsequent processing stages. In the proposed research, we will measure how the surrounding context (long-range interactions and depth contrast) affects stereoacuity, since context has important effects on perceived depth. In addition, we will examine use stereo transparency measurements to estimate the pooling area for disparity signals, as well as how these signals are combined. The experiments in this proposal will examine stereo processing in normal human observers. About 5 percent of the population suffers from oculomotor (strabismus) or refractive (anisometropia) disorders that threaten the development of normal stereopsis. While most individuals can cope with the loss of stereopsis, abnormal binocular development is also frequently associated with a deficit in the acuity of one eye (amblyopia) -- a more serious problem. These studies of normal observers will provide basic knowledge about normal stereopsis, a prerequisite for understanding abnormalities of the binocular system.